Tapping machines, employing shell-type cutters, are commonly used for removing wall sections from water mains, pipelines and the like. The shell cutters which are presently employed commonly include tungsten carbide teeth, brazed into connection with raised portions of the shell cutter.
Conventional shell cutters suffer from several disadvantages. When such cutters are used in the field, there is always the chance that one or more of the brazed teeth will be broken off during use. Once one of the integral teeth is broken off, the remaining teeth may also break, in a chain reaction.
The conventional shell cutter and tapping machine are employed in the field, in conjunction with a gate valve. The gate valve is connected to a portion of pipeline, which is to be tapped. The tapping machine, having the shell cutter, is sealingly connected to the gate valve. It is difficult to make the connection cleanly since the pipeline to be tapped is usually buried and must be excavated. Hence, rock, soil or mud from the excavation can easily become entrapped with the shell cutter after the tapping machine is connected to the gate valve and the pipeline.
Since tapping cutters are attached to gate valves, which are in turn attached to water mains or pipes, the shell cutter is sealed away from view. The operator of the tapping machine does not become aware of any problems in the operation of the shell cutter until a substantial number of the integral teeth have been broken off or damaged in the chain reaction process. Once the teeth have been broken off, it is of course necessary that the cutter be removed; and a substitute cutter be put on in its place. Since the shell cutters are rather large cutting devices (some having diameters of 36 inches or more), it is inconvenient for users of the shell cutter to carry more shell cutters than are necessary to a particular repair or construction site. The repair or construction site is usually removed a substantial distance from shop facilities, thereby making it necessary to carry multiple shell cutters.
Tapping machines are also employed as plate tapping machines. The tapping machine is used to remove a coupon from a flat plate or from a section of metal having a large radius of curvature, such as a oil storage tank. A shell cutter having integrally formed teeth which is used for pipe tapping usually cannot be employed for plate tapping. The pipe tapping shell cutter has a relatively large number of teeth. All of these teeth do not cut a pipe or curved surface simultaneously. However, all of the teeth do engage a flat surface simultaneously. Use of a pipe tapping shell cutter for plate tapping imposes a large load on the tapping machine and in some instances can stop the tapping machine. Therefore, a second shell cutter having a relatively small number of teeth must be used for plate tapping. Thus, it is often necessary for the tapping machine user to purchase and carry about both pipe tapping shell cutters and plate tapping shell cutters.
A second disadvantage which shell cutters having integrally-formed teeth suffer from is that periodic maintenance and repair of the shell cutter is difficult to perform; and is expensive. For large shell cutters (such as the one mentioned above, having a 36-inch diameter), conventional milling machines and sharpening machines are simply too small to sharpen properly the integrally-formed teeth with the shell cutter. Specially built machines must be used to repair and maintain the shell cutters; and, of course, maintenance of the shell cutter by such machines adds considerably to the costs of repair of the shell cutter.
What is needed, then, is a removable tooth which is adapted to be selectively connected and disconnected from a shell cutter. The removable tooth should be small; and should be adapted to be quickly and easily connected to and disconnected from a shell cutter under field conditions.